Rydberg states in molecular aggregates as a route to new optoelectronic materials

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Lyudmila V. Slipchenko of Purdue University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop methods to describe systems that harness light energy for useful applications. One example of a mechanism of this type is photosynthesis in plants, but there are a host of artificial systems that attempt to mimic plants. Usually systems of this type have compact charge distributions, but recent work has shown that states with diffuse charge distributions may also be capable of capturing and using light energy. These states have been shown to exist in a variety of nanomaterials, including graphene sheets, nanotubes and fullerenes, and these states holds promise to develop a new class of materials with superior properties for electronics applications. Dr. Slipchenko and her group are developing theoretical and computational methods for the accurate description of such diffuse states in molecular crystals. Armed with these methods, Dr. Slipchenko is also exploring possibilities for energy and electron transport through these diffuse states in molecular clusters. New methods and algorithms developed in this project are broadly distributed and serve as robust tools to predict the behavior of light-induced phenomena in chemistry, biology, and materials. Dr. Slipchenko serves as a role model for female students and postdocs and provides support and advice in career-life balancing challenges. She also organizes national and international conferences and works to increase the participation and visibility of young female scientists. Dr. Slipchenko and her group develop theoretical and computational methodologies that can be applied for rigorous description of diffuse Rydberg and Rydberg-like states in extended systems such as molecular crystals of relevance to optoelectronic materials and natural organic aggregates motivated by photosynthetic complexes. The methodology is based on a fully embedded quantum mechanics / molecular mechanics (QM/MM) approach, in which the environment (MM part) is described by the sophisticated ab initio-based Effective Fragment Potential (EFP), and the interaction between QM and MM subsystems includes electrostatic, polarization, dispersion and exchange-repulsion terms, all of which are described at a quantum-mechanical level. In this project, Dr. Slipchenko and her group are exploring possibilities for energy and electron transport through utilization of Rydberg states in molecular materials by development of full embedding QM/EFP methods for electronic excited states and development of vibronic coupling models that describe exciton interactions and transport in molecular crystals and other multi-chromophore systems. The project contributes to the research infrastructure by integrating new computer codes in the open-source, freely available libefp library and in the open-source PSI4, GAMESS, NWChem and MOLCAS electronic structure packages, as well as open-source modules in the Q-Chem quantum chemistry software. Web-based interactive software for predicting vibronic interactions and energy/electron transport in molecular aggregates is being developed and is available at the Purdue University NanoHUB. The software can be adapted as a teaching tool in quantum mechanics, spectroscopy and nanotechnology courses.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.